ok...it's a design flaw with the newer iphones...doesn't just affect the 4s but also the 4 as well. Some of them have a defect with the sim tray where metal on it touches the sim card, causing a short circuit and the sim can't be detected.

There's a tutorial on how to put some tape on the sim tray to prevent it from touching. You can try it and see if it works for you. If not, then you would need to have apple repair/replace it.